The endogenous ecotropic virus of the low leukemic strains of mice C3H and BALB/c is poorly infectious and of low pathogenicity. Cells infected with these viruses produce gs antigen and express GCSA (Gross Cell Surface Antigen) on their membranes but not GIX and production of virus particles is exceedingly low. The level of gp70 produced in these cells is 10- to 100-fold below the level found in productively infected cells whereas p30 antigens are present in comparable quantities. Many of the cells that are infected with replication deficient virus segregate gs antigen negative cells in subsequent cell divisions. These poorly infectious viruses can be converted to a replication competent pathogenic form when grown in certain transformed mouse cells. This conversion involves several mutations and possibly recombinational events. The replication competent virus is able to activate xenotropic virus in infected cells and is pathogenic in NFS mice. BIBLIOGRAPHIC REFERENCE: Rapp, U.R. and Nowinski, R.C.: Endogenous ecotropic mouse type C viruses deficient in replication and production of XC plaques. J. Virol. 18: 411-417, 1976.